The present invention relates to a method for manufacturing an optical fiber preform and an apparatus therefor which can manufacture optical fibers exhibiting a reduced transmission loss caused by hydroxyl groups in the region of wavelengths between 1280 nm and 1600 nm.
Hydroxyl groups that are present in glass used in forming optical fiber cause absorption in the wavelength vicinity of 1385 nm, which is one of the causes of transmission loss of the optical fibers.
For this reason, conventionally, a dehydration step has been used in the process for manufacturing an optical fiber preform to reduce hydroxyl groups contained in optical fiber preforms, as taught in Japanese Patent Application, First Publication No. S57-17433.
This dehydration method includes placing a porous glass preform obtained by vapor-phase deposition methods, such as the VAD method or the OVD method, in a dehydration apparatus; supplying a dehydrating agent containing a chlorine-based compound gas, such as Cl2, SOCl2, or CCl4, to this dehydration apparatus; and heating the optical fiber preform at about 1000° C.-1300° C. to remove water absorbed in the porous glass.
However, the effect of dehydration by this conventional dehydration method may be insufficient, and an improvement thereof has been desired.
Such improvements include, for example, methods disclosed in Japanese Patent Application, First Publication No. H11-171575 and Japanese Patent Application, First Publication No. 2002-187733.
However, these methods have shortcomings and are less practical since these methods require special equipment, such as a plasma etching apparatus, for example, or these methods make it difficult to manufacture a large optical fiber preform, or these methods entail complex operating conditions.
Furthermore, other than the above-identified technique, there are a number of related art patents relating to such a dehydrating treatment of optical fiber preforms, including those disclosed in U.S. Pat. No. 6,131,415 and U.S. Patent Application, Publication No. 2002-0073741.
Accordingly, the present invention provides a method for manufacturing an optical fiber preform and an apparatus therefor which can reduce hydroxyl groups in an optical fiber preform to a sufficient level without requiring any special equipment or operating conditions.